Defaults Attachments to empty list.

This allows a more 'careless' usage of the method, preventing an exception `TypeError: 'NoneType' object is not iterable`.
Added to_mime method to Attachments.

Motivation for this was seamless integration with https://github.com/themartorana/python-postmark/ API.
`inbound.attachments(as_mime=True)` would return a list of
attachments compatible with the `PMMail` `attachments` argument.
Added missing import.
Adding simple test cases to MIME functionality.
Merge pull request #7 from serpulga/master

to_mime method. Better python-postmark compat